Journal of Composite Materials http://jcm.sagepub.com
Experimental Study of Dynamic Crack Growth in Unidirectional Graphite/Epoxy Composites using Digital Image Correlation Method and High-speed Photography Dongyeon Lee, Hareesh Tippur, Madhu Kirugulige and Phillip Bogert Journal of Composite Materials 2009; 43; 2081 originally published online Jul 9, 2009; DOI: 10.1177/0021998309342139 The online version of this article can be found at: http://jcm.sagepub.com/cgi/content/abstract/43/19/2081
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Experimental Study of Dynamic Crack Growth in Unidirectional Graphite/Epoxy Composites using Digital Image Correlation Method and High-speed Photography DONGYEON LEE, HAREESH TIPPUR*
AND
MADHU KIRUGULIGE
Department of Mechanical Engineering, Auburn University Auburn, AL, USA
PHILLIP BOGERT NASA Langley Research Center, Hampton, VA, USA
ABSTRACT: In this work, fracture behavior of multilayered unidirectional graphite/epoxy composite (T800/3900-2) materials is investigated. Rectangular coupons with a single-edged notch are studied under geometrically symmetric loading configurations and impact loading conditions. The notch orientation parallel to or at an angle to the fiber orientation is considered to produce mode-I or mixed-mode (mode-I and -II) fracture. Feasibility of studying stress-wave induced crack initiation and rapid crack growth in fiber-reinforced composites using the digital image correlation method and high-speed photography is demonstrated. Analysis of photographed random speckles on specimen surface provides information pertaining to crack growth history as well as surface deformations in the crack-tip vicinity. Measured deformation fields are used to estimate mixed-mode fracture parameters and examine the effect of fiber orientation () on crack initiation and growth behaviors. The samples show differences in fracture responses depending upon the orientation of fibers. The maximum crack speed observed is the highest for mode-I dominant conditions and it decreases with fiber orientation angle. With increasing fiber orientation angle, crack takes longer to attain the maximum speed upon initiation. Continuous reduction of dynamic stress intensity factors after crack initiation under mode-I conditions is attributed to crack bridging. The crack initiation toughness values decrease with the degree-of-anisotropy or increase with fiber orientation angle. A rather good agreement between crack initiation toughness values and the ones from previous investigations is observed. There is also a good experimental correlation between dynamic stress intensity factor and crack-tip velocity histories for shallow fiber orientations of ¼ 0, 15, and 30 .
*Author to whom correspondence should be addressed. E-mail:
[email protected] Figures 37, 9, 11 and 12 appear in color online: http://jcm.sagepub.com
Journal of COMPOSITE MATERIALS, Vol. 43, No. 19/2009 0021-9983/09/19 208128 $10.00/0 DOI: 10.1177/0021998309342139 ß The Author(s), 2009. Reprints and permissions: http://www.sagepub.co.uk/journalsPermissions.nav Downloaded from http://jcm.sagepub.com at AUBURN UNIV on August 25, 2009
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D. LEE ET AL. KEY WORDS: dynamic fracture, mixed-mode crack growth, fiber reinforced composites, optical metrology, digital speckle correlation, high-speed photography, stress intensity factors.
INTRODUCTION RAPHITE/EPOXY COMPOSITES are widely used in aerospace applications [1] for their excellent thermo-mechanical attributes. In view of the vast range of strain-rates this material encounters during service life, it is important to study and quantify mechanical performance of these materials in general and fracture behavior under elevated rates of loading in particular. In this study, primary focus is on stress-wave induced mixed-mode fast fracture in unidirectional graphite/epoxy coupons resulting from low velocity impact. Mixed-mode deformations in unidirectional composites arise when (a) fibers are oriented at an angle in a specimen subjected to configurationally symmetric loading condition, (b) fibers are aligned relative to loading direction, but subjected to configurationally asymmetric loading, or (c) both. To date a number of studies on failure of fiberreinforced composites under mixed loading are reported. Tirosh [2] studied energy release rate in unidirectional composites containing an inclined center crack parallel to fiber orientation. The crack was subjected to mixed-mode conditions in this study by loading at an angle with respect to fiber direction. Donaldson [3] also studied mixed-mode fracture of unidirectional graphite/epoxy system in order to establish a fracture envelope for the material. In this work, as the angle between the crack (parallel to fiber orientation) and loading axis decreased from 90 to 10 , mode-I critical stress intensity factor remained nearly constant, whereas mode-II counterpart increased exponentially. However, for angles
